Kathryn Hopkins

Moderate cochlear hearing loss leads to a reduced ability to use temporal fine structure information

The ability of normally hearing and hearing-impaired subjects to use temporal fine structure information in complex tones was measured. Subjects were required to discriminate a harmonic complex tone from a tone in which all components were shifted upwards by the same amount in Hz, in a three-alternative, forced-choice task. The tones either contained five equal-amplitude components (non-shaped stimuli) or contained many components, but were passed through a fixed bandpass filter to reduce excitation pattern changes (shaped stimuli). Components were centered at nominal harmonic numbers (N) 7, 11, and 18. For the shaped stimuli, hearing-impaired subjects performed much more poorly than normally hearing subjects, with most of the former scoring no better than chance when N=11 or 18, suggesting that they could not access the temporal fine structure information. Performance for the hearing-impaired subjects was significantly improved for the non-shaped stimuli, presumably because they could benefit from spectral cues. It is proposed that normal-hearing subjects can use temporal fine structure information provided the spacing between fine structure peaks is not too small relative to the envelope period, but subjects with moderate cochlear hearing loss make little use of temporal fine structure information for unresolved components.

The importance of temporal fine structure information in speech at different spectral regions for normal-hearing and hearing-impaired subjects

Speech reception thresholds (SRTs) were measured for target and competing-speech signals, processed to contain variable amounts of temporal fine structure (TFS) information. Signals were filtered into 30 1-ERB(N) wide channels (where ERB(N) refers to the bandwidth of normal auditory filters), which were either tone vocoded, preserving temporal envelope information (extracted using the Hilbert transform), or left unprocessed, containing both TFS and envelope information. Improvements in SRT were compared when TFS was progressively introduced, starting with the high- or low-frequency channels. The results suggest some redundancy in the TFS information across frequency regions. In a second experiment, the signal was divided into five, 6-ERB(N)-wide spectral regions, four of which were tone vocoded. The remaining region was either absent (creating a spectral notch) or was present and unprocessed. SRTs were measured for normal-hearing and hearing-impaired subjects. Conditions where all channels were vocoded or unprocessed were also included. Normal-hearing subjects benefited similarly when TFS information was added to each region, suggesting that TFS information is important over a wide frequency range. Hearing-impaired subjects benefited less, although the benefit varied across subjects. Benefit from TFS information in speech was correlated with a psychophysical measure of TFS sensitivity obtained at two center frequencies.

The influence of age and high-frequency hearing loss on sensitivity to temporal fine structure at low frequencies (L).

Sensitivity to temporal fine structure (TFS) at low frequencies may be adversely affected by hearing loss at high frequencies even when absolute thresholds at low frequencies are within the normal range. However, in several studies suggesting this, the effects of hearing loss and age were confounded. Here, interaural phase discrimination (IPD) thresholds for pure tones at 500 and 750 Hz were measured for 39 subjects with ages from 61 to 83 yr. All subjects had near-normal audiometric thresholds at low frequencies, but thresholds varied across subjects at high frequencies. IPD thresholds were correlated with age. IPD thresholds for the test frequency of 750 Hz were weakly correlated with absolute thresholds at high frequencies, but these correlations became non-significant when the effect of age was partialed out. The results do not confirm that sensitivity to TFS at low frequencies is influenced by hearing loss at high frequencies, independently of age.

Development of a fast method for measuring sensitivity to temporal fine structure information at low frequencies

Abstract Recent work suggests that hearing-impaired subjects are relatively insensitive to temporal fine structure (TFS) information, but that sensitivity among subjects varies considerably. 27 developed a fast and easy to administer test of sensitivity to TFS, but it can only be used at medium to high frequencies. Here we describe a binaural method that can be used at lower frequencies. An adaptive two-alternative forced-choice task was used. Each interval contained four tones with frequency f; in one interval all tones were diotic, and in the other tones one and three were diotic while tones two and four had an interaural phase shift, Δϕ. The task was to identify the interval with the phase-shifted tones. For normal-hearing subjects, the effects of sensation level and training on performance were small, and the test could be performed reliably for f = 250, 500, and 750 Hz. Sumario Varios trabajos recientes sugieren que las personas con impedimentos auditivos son relativamente no sensibles a la información temporal de estructura fina (TFS), pero la sensibilidad entre sujetos varía considerablemente. 27 desarrollaron una prueba rápida de sensibilidad TFS fácil de administrar, pero solo puede usarse en las frecuencias medias y altas. Describimos aquí un método binaural que puede usarse en frecuencias graves. Se usó una tarea adaptativa para la selección forzada de dos alternativas. Cada intervalo contenía cuatro tonos con frecuencia f; en un intervalo, todos los tonos fueron dicóticos y con los otros tonos, uno y tres, fueron dicóticos mientras que los tonos dos y cuatro, tuvieron un cambio de fase interaural Δϕ. La tarea fue identificar el intervalo con los tonos que tenían cambio de fase. En los sujetos con audición normal, fueron pequeños los efectos del nivel de sensación y el entrenamiento en el desempeño, y la prueba podría ser realizada confiablemente para f =250, 500 y 750 Hz.

Frequency discrimination of complex tones by hearing-impaired subjects: Evidence for loss of ability to use temporal fine structure.

For normally hearing subjects, thresholds for discriminating the fundamental frequency (F0) of a complex tone, F0DLs, increase when the number of the lowest harmonic, N, is above eight. A previous study showed that F0DLs were affected by component phase for N above 7, and it was argued that the increase in F0DLs with increasing N reflects a loss of temporal fine structure information. Here, subjects with moderate hearing loss were tested in a similar experiment. F0DLs were measured for tones with three successive harmonics, added in cosine or alternating phase. The center frequency was 2000 Hz. N was varied by changing the mean F0. A background noise was used to mask combination tones. F0 was roved across trials and N was roved by +/-1, to reduce use of excitation pattern cues. F0DLs were smaller for cosine than for alternating phase for four out of six subjects, and this occurred once N exceeded 5. In contrast to the result for normally hearing subjects, F0DLs decreased with increasing N. Performance was much worse than obtained for normally hearing subjects at the same center frequency, suggesting that most of the hearing-impaired subjects had a poor ability to use temporal fine structure information.

The effects of age and hearing loss on interaural phase difference discrimination

The discrimination of interaural phase differences (IPDs) requires accurate binaural temporal processing and has been used as a measure of sensitivity to temporal envelope and temporal fine structure (TFS). Previous studies found that TFS-IPD discrimination declined with age and with sensorineural hearing loss (SNHL), but age and SNHL have often been confounded. The aim of this study was to determine the independent contributions of age and SNHL to TFS and envelope IPD discrimination by using a sample of adults with a wide range of ages and SNHL. A two-interval, two-alternative forced-choice procedure was used to measure IPD discrimination thresholds for 20-Hz amplitude-modulated tones with carrier frequencies of 250 or 500 Hz when the IPD was in either the stimulus envelope or TFS. There were positive correlations between absolute thresholds and TFS-IPD thresholds, but not envelope-IPD thresholds, when age was accounted for. This supports the idea that SNHL affects TFS processing independently to age. Age was positively correlated with envelope-IPD thresholds at both carrier frequencies and TFS-IPD thresholds at 500 Hz, when absolute thresholds were accounted for. These results suggest that age negatively affects the binaural processing of envelope and TFS at some frequencies independently of SNHL.

Subcortical neural synchrony and absolute thresholds predict frequency discrimination independently

The neural mechanisms of pitch coding have been debated for more than a century. The two main mechanisms are coding based on the profiles of neural firing rates across auditory nerve fibers with different characteristic frequencies (place-rate coding), and coding based on the phase-locked temporal pattern of neural firing (temporal coding). Phase locking precision can be partly assessed by recording the frequency-following response (FFR), a scalp-recorded electrophysiological response that reflects synchronous activity in subcortical neurons. Although features of the FFR have been widely used as indices of pitch coding acuity, only a handful of studies have directly investigated the relation between the FFR and behavioral pitch judgments. Furthermore, the contribution of degraded neural synchrony (as indexed by the FFR) to the pitch perception impairments of older listeners and those with hearing loss is not well known. Here, the relation between the FFR and pure-tone frequency discrimination was investigated in listeners with a wide range of ages and absolute thresholds, to assess the respective contributions of subcortical neural synchrony and other age-related and hearing loss-related mechanisms to frequency discrimination performance. FFR measures of neural synchrony and absolute thresholds independently contributed to frequency discrimination performance. Age alone, i.e., once the effect of subcortical neural synchrony measures or absolute thresholds had been partialed out, did not contribute to frequency discrimination. Overall, the results suggest that frequency discrimination of pure tones may depend both on phase locking precision and on separate mechanisms affected in hearing loss.

Discrimination of complex tones with unresolved components using temporal fine structure information

The information used to discriminate complex tones with (largely) unresolved components was assessed. In experiment 1, subjects discriminated a harmonic complex tone, H, with fundamental frequency F0, from an inharmonic tone, I, in which all components were shifted upwards by the same amount in hertz. Tones H and I had the same envelope repetition rate but different temporal fine structure (TFS). The tones were passed through a fixed bandpass filter centered on harmonic N, to reduce excitation pattern cues. For all F0s (35-400 Hz), performance decreased as N was increased from 11 to 15, but, except for F0=35 Hz, remained above chance for N=15, where all harmonics should be unresolved. This suggests that discrimination can be based on TFS rather than on partially resolved components. In experiment 2, subjects discriminated the F0 of complex tones filtered as in experiment 1. Here, both envelope rate and TFS cues were available. Except for F0=35 Hz, discrimination thresholds, expressed as the Weber fraction for a change in time interval, were similar to those measured in experiment 1, suggesting that performance in experiment 2 was dominated by the use of TFS rather than envelope cues.

Effect of speech material on the benefit of temporal fine structure information in speech for young normal-hearing and older hearing-impaired participants

Objective: The purpose of this study was to investigate the influence of the type of speech material on the benefit obtained from temporal fine structure (TFS) information in speech for young normal-hearing (YNH) and older hearing-impaired (OHI) participants. Design: The design was based on the work of Hopkins et al. (2008). They measured the speech reception thresholds for a target talker in a background talker as a function of the frequency range over which TFS information was available. The signal was split into 32 channels, each with a bandwidth equal to the equivalent rectangular bandwidth of the “normal” auditory filter at the same center frequency. Above a cutoff (CO) channel, channels were vocoded and contained only temporal envelope information. Channels up to and including CO were not processed. Hopkins et al. found that, as CO was increased, speech reception thresholds decreased more for normal-hearing participants than for participants with cochlear hearing loss, suggesting that the latter were less able to use TFS information. We used the same design, but compared results when the target speech materials were open-set sentences, as used by Hopkins et al., and when they were more predictable sentences with a closed word set (Danish Dantale 2). Results: With the open-set material, YNH listeners benefited more from TFS information than OHI listeners, replicating Hopkins et al. (2008). For the YNH participants, the benefit of adding TFS was greater for the open-set material than for the closed-set material, while no difference in TFS benefit across speech materials was found for the OHI participants. Conclusions: The choice of speech material is important when assessing the benefit of TFS. Several factors may facilitate recognition in the absence of TFS cues, including small set size, predictable temporal structure of the target speech, and contextual effects. We speculate that TFS information is useful for reducing informational masking, by providing cues for the perceptual segregation of the target and background. When the target speech is highly predictable, informational masking may be minimal, rendering TFS cues unnecessary.

Phase locked neural activity in the human brainstem predicts preference for musical consonance.

When musical notes are combined to make a chord, the closeness of fit of the combined spectrum to a single harmonic series (the ‘harmonicity’ of the chord) predicts the perceived consonance (how pleasant and stable the chord sounds; McDermott, Lehr, & Oxenham, 2010). The distinction between consonance and dissonance is central to Western musical form. Harmonicity is represented in the temporal firing patterns of populations of brainstem neurons. The current study investigates the role of brainstem temporal coding of harmonicity in the perception of consonance. Individual preference for consonant over dissonant chords was measured using a rating scale for pairs of simultaneous notes. In order to investigate the effects of cochlear interactions, notes were presented in two ways: both notes to both ears or each note to different ears. The electrophysiological frequency following response (FFR), reflecting sustained neural activity in the brainstem synchronised to the stimulus, was also measured. When both notes were presented to both ears the perceptual distinction between consonant and dissonant chords was stronger than when the notes were presented to different ears. In the condition in which both notes were presented to the both ears additional low-frequency components, corresponding to difference tones resulting from nonlinear cochlear processing, were observable in the FFR effectively enhancing the neural harmonicity of consonant chords but not dissonant chords. Suppressing the cochlear envelope component of the FFR also suppressed the additional frequency components. This suggests that, in the case of consonant chords, difference tones generated by interactions between notes in the cochlea enhance the perception of consonance. Furthermore, individuals with a greater distinction between consonant and dissonant chords in the FFR to individual harmonics had a stronger preference for consonant over dissonant chords. Overall, the results provide compelling evidence for the role of neural temporal coding in the perception of consonance, and suggest that the representation of harmonicity in phase locked neural firing drives the perception of consonance.